The invention relates to a plasma melting furnace comprising a cover and a melt container through the side wall of which plasma burners, arranged so as to be inclined towards the furnace axis are guided. A main flue gas conduit equipped with a draft regulating damper is connected to the cover, and a separate discharge means for flue gases from other furnace openings also are provided.
In a plasma melting furnace an electric discharge is maintained between the plasma burner(s) constituting the cathode(s) and a bottom electrode or melt as anode. The direct current generates what is called a plasma jet, flowing in an electrically conducting channel of ionized gas introduced via the plasma burners. Preferably, nitrogen is used as the gas for ignition, and a noble gas, such as argon, or mixtures of these gases, are used for the melting procedure. Thus, in the interior of the plasma furnace a protective gas atmosphere is created at the same time, which is to be maintained during the melting procedure with as few admixtures as possible. This is particularly required when melting steel qualities with low nitrogen contents.
When melting contaminated scrap, e.g., oil-containing scrap, a heavy smoke gas development takes place every time, the smoke gases usually being discharged or evacuated via a smoke gas socket in the cover. During evacuation the protective gas also is drawn off and, due to the resulting depression, ambient air and thus nitrogen, oxygen and steam will penetrate into the furnace interior through furnace openings, such as the furnace door and the casting spout. This results in contaminations of the melt, which, after casting, are present as inclusions, such as oxides and nitrides, or as dissolved substances, in the cast piece, thus causing a reduced quality of the same.
From U.S. Pat. No. 3,533,611 a ventilation system for electric arc furnaces is known, by means of which gases and fumes are to be removed and desired atmospheric conditions in the furnace are to be created.
Such an electric arc furnace comprises two separated gas-evacuation conduits for the flue gases, each conduit being provided with a draft regulating damper. Furthermore, a selective switch is provided according to U.S. Pat. No. 3,533,611, which opens and closes the draft regulating dampers via valves alternately during the cycles of melting on the one hand and of refining and slagging on the other hand. The electrodes are led through the furnace arch and the maintenance of a protective gas atmosphere in the furnace interior is not provided. The ventilation system according to U.S. Pat. No. 3,533,611, and in particular the evacuation of the fumes rising from the casting spout and the furnace door, is thus made feasible in a very imperfect manner.